Inspection of electronic parts such as such as wafers, circuit boards, flat panel displays, multi-chip modules, and high-density electronic packages requires sufficient camera signal to noise ratio (S/N) to image the part, a fast camera clock rate to obtain good throughput, and sharp focus with high resolution to insure detection of small defects.
Camera signal output is given by the equation:                                                                         Cam                sig                            =                            ⁢                                                                    Illumination                    ⁢                                                                                  ⁢                    Power                                                        #                    ⁢                                                                                  ⁢                    cam                    ⁢                                                                                  ⁢                    pixels                                                  ×                OE                ×                                                                                                      ⁢                              Ref                ×                Integration                ⁢                                                                  ⁢                time                ×                                  NA                  2                                                                                        Equation        ⁢                                  ⁢        1            where:    Illumination Power=total illumination power incident on the part    OE=efficiency or transmittance of the optical system Integration time=time duration over which light is collected by the camera    # cam pixels=the total number of camera pixels    NA2=the numerical aperture of the imaging optics    Ref=the percent of illuminated light reflected off the material for non-fluorescent applications, given by the equation:       reflected    ⁢                  ⁢    light    ⁢                  ⁢    intensity        illumination    ⁢                  ⁢    light    ⁢                  ⁢    intensity      Ref=the percent of fluorescent light emitted by the material in response to the excitation illumination of a different frequency for fluorescent applications; given by the equation:       fluorescent    ⁢                  ⁢    emission    ⁢                  ⁢    light    ⁢                  ⁢    intensity        Illumination    ⁢                  ⁢    excitation    ⁢                  ⁢    light    ⁢                  ⁢    intensity  
The part can be inspected using a single camera or multiple cameras, with each camera viewing a different region of the part, to increase throughput. Each camera may have one or multiple outputs. Therefore, in general terms:    # Cam pixels=total # outputs×# pixels per output
The shortest integration time, in Equation 1, is equal to the time required to read the single longest output. If the number of pixels for each output is identical, the integration time can be expressed as:                               Integration          ⁢                                          ⁢          time                =                              #            ⁢                                                  ⁢            cam            ⁢                                                  ⁢            pixels                                #            ⁢                                                  ⁢            outputs            ×            Ck                                              Equation        ⁢                                  ⁢        2            where Ck=data rate of one output in pixels/second    Substituting Equation 2 into Equation 1 and simplifying terms yields:                               Cam          sig                =                                            Illumination              ⁢                                                          ⁢              Power                                      #              ⁢                                                          ⁢              outputs              ×              Ck                                ×          OE          ×          Ref          ×                      NA            2                                              Equation        ⁢                                  ⁢        3            The signal to noise ratio of the camera (S/N) is given by the equation:S/N=Camsig0/N  Equation 4Where N is the electronic noise of the camera.
The time required to readout all the camera data dictates the speed of the inspection. Multiple outputs enable multiple pixels to be read out simultaneously. The effective readout time per pixel referred to as the effective data rate or speed is given by the equation:Speed=Effective data rate=# outputs×Ck  Equation 5To summarize:Speed=# outputs×Ck  Equation 6                              Cam          sig                =                                            Illumination              ⁢                                                          ⁢              Power                                      #              ⁢                                                          ⁢              outputs              ×              Ck                                ×          OE          ×          Ref          ×                      NA            2                                              Equation        ⁢                                  ⁢        7            S/N=Camsig /NEquation 8
Equations 6 and 7 show that the camera signal is inversely proportional to speed. As camera speed increases, signal output decreases because the time during which the pixel collects light is decreased. In applications where illumination power is not limited, power can be increased to compensate for signal loss due the increase in speed. However, in low camera light conditions, which occur in most fluorescent inspection applications or for inspection of low reflectivity parts, both of which correspond to small values of Ref in Equation 7, it may not be feasible, practical, or cost effective to increase illumination power. Fluorescent inspection systems, such as the Orbotech Blazer or Beltronics Mscan III, use expensive lasers as the illumination source. In such systems, it is not cost effective or practical to increase the size of the laser. In extremely fast non-fluorescent scan applications, bulb life also becomes an issue for high power illumination systems. For example, a frequently used 150-watt halogen lamp (model EJV manufactured by Sylvania) or a typical high-pressure mercury vapor lamp (model XXX manufactured by Osram) are only rated for 200 hours of operation at the rated operating voltages. However, if the EJV lamp is operated at a reduced power its life can be greatly extended. Therefore, inspection systems in which camera light is limited because:                fluorescence is used to image the part,        material reflectivity is low, and        illumination intensity is reduced to increase bulb life.        
The camera signal can be greatly increased by increasing the numerical aperture (NA) of the imaging optics. Equation 7 states that camera signal strength is proportional to NA2. However, the optical depth of focus (DOF) given by the equation:DOF=λ/2×NA2  Equation 9Where: λ=Wavelength of light imaged onto the camera new line decreases inversely as NA2 increases. Therefore, if signal strength is increased by using higher NA optics, it may not be possible to maintain focus as the part is scanned. FIG. 1 in the drawings shows the depth of focus ray diagram for a low S1 and high S2 NA lens. The cone angle over which light is collected is given by the equation:Light Collection Cone Angle θ=2×sin1(NA).  Equation 10
Table 1 is a list of commercially available lenses from the Zeiss Corporation. The table lists the depth of focus, numerical aperture, resolving power, light collection coefficient, light collection cone angle, working distance magnification and part number for each lens.
TABLE 1Commercially Available Objective Lenses From ZeissResolvingDepth ofPower forFocusLightLight Collectionλ = 0.55λ = 0.55Magnification/NumericalCollectionCone Angle(Microns)(Microns)Workingpixel size(microns)Zeiss PartNumberAperture(NA)Coefficient(NA2)(degrees)θ = 2 × sin3(NA)  λ      2    ×    NA    λ      2    ×          NA      2      Distance(WD)1.25×/10.44423000.0350.00124.07.82293.9 mm2.5×/5.24423100.0750.00568.63.6499.4 mm5×/2.64403200.150.0225171.812.213.6 mm 5×/2.6—0.250.0625291.14.4—10×/1.34428320.250.0625291.14.412.7 mm 10×/1.34423300.300.0900350.93.15.7 mm20×/0.654428400.400.1600470.71.79.8 mm10×/1.34401350.500.2500600.51.12.0 mm20×/0.654423400.500.2500600.51.11.4 mm
Note that in Table 1 the 1.25X lens with an NA of 0.035 has a depth of focus of 229 microns whereas the 20X lens with an NA of 0.50 only has a depth of focus of 1.1 microns. However, for a through the lens illumination system as shown in FIG. 2, the 20X 0.5 NA lens collects 204 times (0.5/0.035)2 more light than the 1.25X 0.035 NA lens. Unfortunately, unless focus can be maintained to within the 1.1 micron depth of focus, the 20X 0.5 NA lens cannot be used to inspect the part. With current wafers 300 mm in diameter (300,000 microns) and circuit boards or flat panel displays over 700 mm (700,000 microns), maintaining focus to within microns becomes very difficult. Therefore, many inspection systems are forced to use low NA optics, which limits their ability to inspect low reflectivity and fluorescent parts at high speeds. As another example, consider the 5X 0.15 NA and the 5×0.25 NA lens, both with the same magnification. The 0.25 NA lens collects 2.7 times more light than the 0.015 NA lens, but only has a depth of field of 4.4 microns as compared to 12.2 microns. While the 0.25 NA lens enables the system to operate 2.7 times faster than the 0.15 NA lens, this lens can only be used if focus can be maintained to within 4 microns.
Another advantage of high NA optics is higher resolution. Resolving power is given by the equation:                               Resolving          ⁢                                          ⁢          power                =                  λ                      2            ×            NA                                              Equation        ⁢                                  ⁢        11            where λ=wavelength of imaged light.
Therefore, the 1.25X 0.035NA lens can only resolve 7.8 microns as compared to the 20X 0.5 NA lens which can resolve 0.5 microns for λ=0.55 microns. However, high NA optics cannot be used unless focus can be maintained as the part is scanned.
Therefore, many inspection systems, which do not have auto focus capability, are forced to use low NA optics to maintain focus and are unable to inspect very small features that require high magnification and high resolution.
Two types of auto-focus systems designed for automated inspection applications were introduced by Beltronics in its Mscan II and Mscan III inspection Systems. However, these systems are not suited for high-speed inspection applications because they are slow, with focus time being proportional to the size and flatness of the part. Both auto-focus methods are limited to parts whose surfaces can be topographically expressed as a small number of contiguous planes, as explained with the aid of FIG. 3.
FIG. 3 shows a wavy rolling part with shallow sloped peaks and valleys. Projected onto the part, for the purpose of explaining the focus algorithm, is a grid divided into 16 points, which defines 9 contiguous planes used to model the topology of the part. This group of contiguous planes thus defines a focus map. During the inspection process, the imaging optics is adjusted to track the focus map as a function of the X and Y position of the scanning stage. The computation of this focus map is a slow process, which is performed prior to scanning each part for defects. It involves moving to each point on the grid and computing the Z-axis position that yields optimal focus.
The Beltronics Mscan II defines optimal focus as the Z position that maximizes spatial high frequency information in the image. It is used to inspect grainy textured metal masks. However, this method is not applicable for inspection of smooth homogeneous materials such as circuit board and wafer substrates and resists, which do not contain significant grain, texture, or high frequency information.
The Mscan III defines optimal focus as the Z position that superimposes two slits onto the surface of the part, one projected from the right, the other from the left, as shown in FIG. 4. This technique has two major limitations When the magnification is high corresponding to a small field of view (FOV), small variations in focus height project the slits totally out of the FOV leaving nothing to focus on, as shown in FIG. 4. When the slits are in the FOV, this technique does not indicate which direction to move along the Z-axis. When the slits are projected out of the FOV, the time required to find optimal focus greatly increases because. the focus algorithm cannot determine in which direction to move.
As an example, let the part be positioned at the Z position labeled “Low” in FIG. 4. No slits are present in the FOV. If the next guess is to move downward such that the optics is focused at position “Very Low” the FOV will still be blank. If the next guess is to move upward such that the optics is now in focus at position “High” the FOV is still be blank. Therefore, one cannot determine in which direction to move and the system hunts up and down until an image of the slits appears in the FOV. As a result of this hunting, the algorithm is easily confused and can become very slow. As parts become larger in size and inherently less flat or as features become smaller requiring higher magnifications for inspection, more points are required in the focus map (FIG. 3) to model the part. This drastically increases the time required to compute the entire focus map such that this computation may take more time than the actual inspection process, which occurs immediately after the map has been calculated.